This invention relates to formulations comprising inorganic oxide particles. In particular, this invention relates to ink receptive coating formulations for paper comprising a novel porous, fine inorganic oxide which provides excellent ink absorption properties, and if desired, glossy finishes.
Ink receptive coatings typically contain various proportions of inorganic pigments and binder(s). The proportions of these components affect the properties of these coatings, e.g., ink absorption properties. One means of characterizing the proportion of inorganic pigment relative to the proportion of binder is by the pigment volume concentration, or PVC. The definition of pigment volume concentration (PVC) is: 100*Vp/(Vp+Vb), where Vp is the volume of the pigment and Vb is the volume of the binder.
When a coating is formulated at low PVC, the binder constitutes the continuous phase of the coating within which pigment particles are dispersed. When a coating is formulated at high PVC, the binder phase is no longer a continuous phase, that is, there is not enough binder to fill the voids between packed and semi-rigid or rigid pigment particles. The proportion at which the binder is no longer considered the continuous phase is referred to in the art as the critical pigment volume concentration (CPVC). At proportions above CPVC, a network of interparticle pores form between the close packed particles, and these interparticle pores become reservoirs for ink that is subsequently applied to the dried coating.
It also is well known in the art that glossy and permeability properties of coatings comprising inorganic pigment and binder depend on the PVC. See Outlines of Paint Technology 3rd ed., W. M. Morgans, Halsted Press (a division of John Wiley and Sons, Inc.) New York, N.Y., 1990, p.7. When a coating is formulated below the CPVC, a matte effect is created by imparting surface roughness and can be generally attained when the pigment particle size is large when compared to the coating thickness. Relatively glossy coatings can be achieved when the pigment particles are small when compared to the coating thickness. Thus, matte finishes and coating gloss can be controlled by judicious choice of pigment particle size in relation to the coating film thickness. However, unless the binder is hygroscopic, such coatings will be relatively impermeable to water. For an ink-jet printing application with aqueous inks, such a coating would suffer from the shortcoming of relatively long ink dry-time. A key attribute of ink-receptive coatings is the ability to absorb the ink fluid rapidly so that the image becomes fixed to the media as quickly as possible. This minimizes smudging.
One known formulation comprising hygroscopic binders is an ink-receptive coating formulated at low PVC using colloidal silica as the pigment in conjunction with hygroscopic binders such as PVOH. These formulations generally result in relatively glossy coatings and the hygroscopic binders absorb moisture via partial solubility of the ink-fluid. The colloidal silica in this instance serves to modify the coating properties to improve the image characteristics once the coatings is printed. However, colloidal silica is non-porous and coatings prepared from colloidal silicas are relatively dense. Such a coating therefore lacks capacity to absorb large quantities of liquid ink. Furthermore, ink-drytimes are relatively slow.
When coatings are formulated at high PVC (and specifically, above the CPVC), it is typical to observe very different gloss and moisture permeability properties than when formulated below the CPVC. At high PVC, a network of interstitial void space, or pores between the particles, is created by imperfect packing of the particles and the lack of a suitable amount of binder to fill the interstitial voids. Such coatings tend to display a high degree of moisture permeability because of liquid flow through the interstitial pores, which is desirable for the ink-receptive coating application. However, the gloss of such coatings is usually relatively low because the surface exhibits a degree of roughness that is related to the pigment particle size. It is possible to obtain relatively glossy coatings in this system, but relatively small pigment particles are necessary and typically relatively non-porous particles are used. For example, colloidal silica described in European Patent Application 803,374 can be used for glossy applications. Colloids derived from fumed silica also have been used for this purpose, but those materials are also non-porous.
Other pigments such as clays, aluminas, diatomaceous earth, precipitated silicas, etc., disclosed in U.S. Pat. Nos. 4,460,637 and 5,030,286, are used as well. While some of these pigments do have internal porosity, that porosity is subject to substantial reduction when the coating containing the pigments is applied and dried.
Accordingly, there is a need to provide pigments for glossy ink receptive coatings in which the inorganic particles contain internal porosity, regardless of the coating PVC. It also is desirable to have pigment porosity and hence coating porosity which is not as affected by external factors, such as shear, and is reliably present even after the pigment and coating is processed and dried.
The formulations of this invention comprise binder and porous inorganic oxide particles or pigments which have a median particle size in the range of 0.05 to about 3 microns. Unlike prior art colloidal particles, the particles of this invention have a porous structure such that at least about 0.5 cc/g of the pore volume is from pores having a pore size of 600 xc3x85 or less. Porosity from pores less than 600 xc3x85 is referred to herein as internal porosity, i.e., porosity present in the particles themselves. Indeed, the internal porosity is reflected by a xe2x80x9cviscosity derived pore volumexe2x80x9d, defined later below, of at least about 0.5 cc/g. There also are embodiments comprising silica gel particles in which at least about 0.7 cc/g and at least 0.9 cc/g of pore volume is from pores having sizes less than 600 xc3x85. In these embodiments, at least 80% of the pore volume is from pores having pore sizes less than 300 xc3x85.
The internal porosity of the particles in this invention is relatively stable and unlike prior art precipitated silicas is less susceptible to complete collapse under capillary pressures created when the water evaporates from the dispersion during drying.
The formulations can be coated onto substrates and dried to form a porous layer which is particularly suitable as an ink receptive layer, e.g., ink jet paper. The dried layer resulting from the formulation generally has good ink absorption properties. Embodiments comprising particles having a median particle size in the range of 0.05 to 1 micron can be used to prepare relatively high gloss finishes, particularly for photorealistic printing.